Antigens are potentially harmful molecules, and antigen processing refers to the biological method of identifying these molecules and dealing with those that are dangerous to the body. Antigens produced by the body are typically left alone, while invading molecules foreign to the body are usually neutralized or destroyed. There are two ways antigens are processed in the body, and each involves different cells of the immune system. The endogenous pathway manages particles inside cells, and the exogenous pathway is in charge of unfamiliar particles outside cells.
The endogenous pathway of antigen processing mainly handles viruses or proteins made by the body and known as self-antigens. Inside a cell, the virus or self-antigen is broken down into fragments and introduced to Class I major histocompatibility complex (MHC) molecules. MHC molecules connect with the antigen, transport it to the cell membrane and present it to T-cells in the blood. During this antigen presentation stage, self-antigens are ignored by T-cells, but virus particles trigger a response. To prevent a virus from spreading, cytolytic T-cells eliminate infected cells.
The exogenous pathway of antigen processing attends to unfamiliar particles in the blood. An antigen-presenting cell (APC) consumes the foreign invader, typically a bacterium, through the process of endocytosis. Examples of APCs include macrophages, B-cells and dendritic cells. Inside the APC, the foreign particles are housed inside a vesicle where they are broken down into fragments. When this vesicle merges with another vesicle filled with Class II MHC molecules, the MHC molecules link up with the fragments.
The combined vesicle moves to the cell membrane for antigen presentation in which the MHC molecules present the antigens to T-cells in the blood. Instead of cytolytic T-cells, helper T-cells interact with antigens presented on APCs. Helper T-cells assist B-cells to produce antibodies against bacteria. When antibodies are created for a specific type of bacteria, any future offenders of the same kind are more quickly recognized by the immune system. This triggers a polyclonal response in which many B-cells produce a lot of antibodies against the same antigen.
Antigen processing can be harmful to the body if a person has an autoimmune disease, such as celiac disease or rheumatoid arthritis. In a person with an autoimmune disease, the body incorrectly identifies self-antigens as foreign invaders and attacks them. Allergic reactions are also caused by an inappropriate immune system response in which the immune system overreacts to antigens, causing inflammation.